Bug 640444: Self-copies end up changing all buffer content, so we need to read back the entire buffer after swapping. r=Bas a=b

gfx/layers/ThebesLayerBuffer.cpp

@@ -222,6 +222,7 @@ ThebesLayerBuffer::BeginPaint(ThebesLayer* aLayer, ContentType aContentType,
                               PRUint32 aFlags)
 {
   PaintState result;
+  result.mDidSelfCopy = PR_FALSE;
   float curXRes = aLayer->GetXResolution();
   float curYRes = aLayer->GetYResolution();
 
@@ -336,6 +337,7 @@ ThebesLayerBuffer::BeginPaint(ThebesLayer* aLayer, ContentType aContentType,
           nsIntRect srcRect(nsIntPoint(0, 0), mBufferRect.Size());
           nsIntPoint dest = mBufferRect.TopLeft() - destBufferRect.TopLeft();
           MovePixels(mBuffer, srcRect, dest, curXRes, curYRes);
+          result.mDidSelfCopy = PR_TRUE;
           // Don't set destBuffer; we special-case self-copies, and
           // just did the necessary work above.
           mBufferRect = destBufferRect;

gfx/layers/ThebesLayerBuffer.h

@@ -110,12 +110,14 @@ public:
    * by ThebesLayerBuffer and must be redrawn on the screen.
    * mRegionToInvalidate is set when the buffer has changed from
    * opaque to transparent or vice versa, since the details of rendering can
-   * depend on the buffer type.
+   * depend on the buffer type.  mDidSelfCopy is true if we kept our buffer
+   * but used MovePixels() to shift its content.
    */
   struct PaintState {
     nsRefPtr<gfxContext> mContext;
     nsIntRegion mRegionToDraw;
     nsIntRegion mRegionToInvalidate;
+    PRPackedBool mDidSelfCopy;
   };
 
   enum {

gfx/layers/basic/BasicLayers.cpp

@@ -428,6 +428,7 @@ protected:
               const nsIntRegion& aRegionToDraw,
               const nsIntRegion& aExtendedRegionToDraw,
               const nsIntRegion& aRegionToInvalidate,
+              PRBool aDidSelfCopy,
               LayerManager::DrawThebesLayerCallback aCallback,
               void* aCallbackData)
   {
@@ -608,6 +609,7 @@ BasicThebesLayer::PaintThebes(gfxContext* aContext,
       SetAntialiasingFlags(this, state.mContext);
       PaintBuffer(state.mContext,
                   state.mRegionToDraw, extendedDrawRegion, state.mRegionToInvalidate,
+                  state.mDidSelfCopy,
                   aCallback, aCallbackData);
       Mutated();
     } else {
@@ -1800,6 +1802,7 @@ private:
               const nsIntRegion& aRegionToDraw,
               const nsIntRegion& aExtendedRegionToDraw,
               const nsIntRegion& aRegionToInvalidate,
+              PRBool aDidSelfCopy,
               LayerManager::DrawThebesLayerCallback aCallback,
               void* aCallbackData);
 
@@ -1860,21 +1863,27 @@ BasicShadowableThebesLayer::PaintBuffer(gfxContext* aContext,
                                         const nsIntRegion& aRegionToDraw,
                                         const nsIntRegion& aExtendedRegionToDraw,
                                         const nsIntRegion& aRegionToInvalidate,
+                                        PRBool aDidSelfCopy,
                                         LayerManager::DrawThebesLayerCallback aCallback,
                                         void* aCallbackData)
 {
   Base::PaintBuffer(aContext,
                     aRegionToDraw, aExtendedRegionToDraw, aRegionToInvalidate,
+                    aDidSelfCopy,
                     aCallback, aCallbackData);
   if (!HasShadow()) {
     return;
   }
 
   nsIntRegion updatedRegion;
-  if (mIsNewBuffer) {
+  if (mIsNewBuffer || aDidSelfCopy) {
     // A buffer reallocation clears both buffers. The front buffer has all the
     // content by now, but the back buffer is still clear. Here, in effect, we
     // are saying to copy all of the pixels of the front buffer to the back.
+    // Also when we self-copied in the buffer, the buffer space
+    // changes and some changed buffer content isn't reflected in the
+    // draw or invalidate region (on purpose!).  When this happens, we
+    // need to read back the entire buffer too.
     updatedRegion = mVisibleRegion;
     mIsNewBuffer = false;
   } else {